Engine liquid fuel supply system



Oct. 2, 1962 A. JUBB ETAL ENGINE LIQUID FUEL SUPPLY SYSTEM Filed Jan. 24, 1958 2a zwz awq az United States Patent Ofiice 3,056,259 Patented Oct. 2, 1962 UPS-5,259 ENGiNE LlQUil) FUEL SUPELY SYSTEM Albert .luhh, Alvaston, and Christopher Linley Johnson,

Allestree, England, assignors to Rolls-Royce Limited,

Derby, England, a British company Filed Jan. 24, USS, Ser. No. 711,036 Claims priority, application Great Britain Jan. 28, 1957 4 Claims. (Cl. oil-39.28)

This invention relates to a liquid fuel system for an engine, such for instance as a gas-turbine engine, having a combustion chamber into which a continuous flow of fuel is delivered in operation of the engine.

In such an engine, there is usually provided primary (or pilot) fuel injector means and secondary (or main) fuel injector means, and the maximum quantity of fuel delivered through the primary fuel injector means is a small proportion of the total fuel delivered to the combustion chamber under maximum fuel flow operating conditions.

This invention has for an object to provide a simple fuel system which caters for a wide range of fuel flows whilst obtaining good atomization of the fuel in the combustion chamber. It will be appreciated that good atomization is desirable in order to maintain combustion, and difficulties have been experienced with known fuel systems in maintaining combustion at relatively low fuel flows such as are necessary under some operating conditions, for instance such as are necessary to satisfy engine requirements when a gas-turbine engine is operating at high altitudes.

According to the present invention, a fuel system for an engine having a combustion chamber to which fuel is delivered continuously in operation, comprises primary and secondary fuel injectors, a main pump with control means metering the total fuel delivery to the fuel injectors by the main pump, a main fuel line receiving the total fuel delivery, branch fuel lines leading from the main fuel line to the primary and secondary fuel injectors respectively, and a pressurizing arrangement, including a pump mechanically driven from the engine, in the branch line leading to the primary injector or injectors whereby a supply of fuel at adequate pressure for atomization is maintained to the primary injector or injectors at all total fuel flowsv Said pressurizing arrangement may also include means to limit the maximum delivery pressure to the primary injectors.

The pressurizing arrangement may comprise a posifive-displacement pump driven mechanically from the engine and having a by-pass fitted with a relief valve to open when the delivery pressure reaches a selected value, or a pump of the pressure compensated kind driven mechanically from the engine.

According to a preferred feature of the invention a non return valve is incorporated in the branch fuel line leading to the secondary fuel injectors. The provision of such a non-return valve prevents air which may be contaminated with carbon being drawn in from the combustion chamber when the pressure on the inlet side of the pressurizing pump is less than the pressure within the combustion chamber.

According to another preferred feature of the invention, there is provided means whereby, when the metered total fuel delivery into the main fuel line is less than the rate of delivery of the pump of the pressurizing arrangement in the branch line to the primary injector or injectors, a quantity of fuel is circulated in the pump of the pressurizing arrangement, thereby to avoid cavitation. One fuel system according to this feature of the invention comprises a non-return valve in the branch line to the secondary fuel injector or injectors, a recirculating by-pass between the inlet and outlet of the pump of the pressurizing arrangement, a valve in the by-pass, and pressure-responsive means operating the valve, the pressure-responsive means being responsive to the pressure in the combustion chamber and to the pressure at the inlet to the pump and being arranged to open the valve When the combustion chamber pressure differs from the pump inlet pressure by a selected amount.

One form of fuel system of this invention will now be described with reference to the accompanying diagrammatic drawing.

The fuel system illustrated is suitable for an aircraft gas-turbine jet propulsion engine. I

The engine illustrated comprises a compressor 40 which draws in air from atmosphere and delivers the compressed air to combustion equipment 4-1 wherein fuel is burnt with the air and from which the combustion gases are delivered to a turbine 42. The exhaust gases from the turbine 42 pass into a jet pipe 43 and flow to atmosphere through a propelling nozzle 44 which is shown as a variable area nozzle.

The fuel system for delivering fuel to the combustion equipment 41 comprises a fuel tank 9, a main fuel pump 10, shown as a centrifugal pump, having a suction pipe 111 leading from the fuel tank and a main delivery pipe 12 having connected in it a fuel-cooled oil cooler 13 through which the fuel from the pump it) is passed, a control mechanism 14 of any known or convenient kind by which the delivery of fuel from pump it} into the main fuel line 12 is metered to suit the total engine fuel requirements, which vary according to the desired engine rotational speed, the altitude of operation and other factors, primary (or pilot) fuel injector means 15, secondary fuel injector means 16 and a system by which the total fuel flow to the injector means 15, 16, is divided between them. The fuel pump 10 may be driven by the engine, or may be driven as indicated by an air turbine 8 which is supplied with operating air from the engine compressor 4-6 through conduit 45 or by any other convenient drive mechanism.

The fuel dividing system comprises branch lines 17a, 1712 leading respectively to a manifold 46 for the primary fuel injector means 15 and to a manifold 47 for the secondary injector means 16.

The branch line 17a has fitted in it an auxiliary pump 18 and a shut-off cock 19. The pump 18 is shown as a gear-type positive-displacement pump and it is driven from the engine through a mechanical drive indicated at 20. The pump 18 has associated with it means to limit to a selected maximum value the pressure rise across the pump and this means is shown as a by-pass conduit 21 containing a spring-loaded pressure relief valve 22 which opens when the delivery pressure reaches the desired value abovethe pressure at the inlet of the pump 18. The pump 18 also has associated with it a fuel re-. circulating by-pass conduit 23, the purpose of which will be described below.

The branch line 17b has fitted in it a spring-loaded non-return valve 24 which permits fuel to flow towards the secondary fuel injector means 16, and a shut-01f cock 25 which may be operatively coupled with shut-off cock 19 as indicated at 26.

The recirculating by-pass 23 has connected in it "a valve comprising a valve body 27, and a piston valve element 28 having two lands 28a, 28b dividing the body into three chambers 29, 3t}, 31, of which chamber 31 contains a spring 32 urging the piston valve to a position in which flow through the portion of conduit 23 leading from the outlet of pump 18 into the valve body 27 is cut off. Chamber 29 is sealed off from chambers 30, 31 by the land 28a and is connected by conduit 33 to the branch line 17b at a point downstream of the nonreturn valve 24. The chamber 34) is connected by a drill- 3 ing 34 to chamber 31. It will be clear that when the piston valve moves sufliciently far to the right the two portions of conduit 23 are joined through chamber 30 so connecting the inlet and outlet of pump 18. The spring 32 may alternatively be housed in the chamber 29.

In operation of the fuel system, fuel metering is effected prior to division of the fuel between the injector means 15' and 16, and an adequate pressure of the fuel entering the combustion chamber through the primary injector means 15 to ensure good atomization is maintained under all operating conditions by the pump 18 At low fuel flows, the suction elfect of pump 18 prevents the pressure in the main pipe line 12 from eXceeding that necessary to open non-return valve 24 and so the whole of the fuel is delivered to the primary injector means 15. As the fuel flow increases so that pressure on the delivery side of the pump 18 increases until at a preselected value of the pressure, the relief valve 22 opens so preventing any further increase of this pressure and thus of the rate of fuel delivery through the primary injector means 15 (which in eifect is a restriction). Any further increase in the total fuel flow is accompanied by an increase in pressure in the main fuel line 12 so that non-return valve 24 opens permitting the additional fuel to flow to the secondary injector means 16.

Under some conditions of operation, for example at high altitude, the total engine fuel requirements may be so low that the pump 18 tends to deliver more fuel than is delivered into the main delivery pipe 12 so that there is a tendency for cavitation to occur in the pump 18. Such cavitation is avoided by permitting recirculation of a quantity of fuel in a path including the pump 18 and the recirculation by-pass 23.

Opening of the valve 27, 28 to permit said recirculation is effected automatically as follows. When cavitation is likely to occur, there is no flow in branch line 17b, and the combustion chamber pressure acts on the piston valve through the column of liquid fuel in branch line 17b and displaces the piston valve 28 to the right against the action of the inlet pressure of pump 18 plus the load of spring 32 in the arrangement shown (or less the spring load when the spring is in chamber 29) until the portions of the conduit 23 are joined through chamber 30, when recirculation occurs.

Since, when the spring 32 is in chamber 29, the inlet pressure of pump 18 necessary to close valve 28 will be higher than the combustion chamber pressure, it will be necessary to employ a higher spring loading of the non-return valve 24, than when the spring 32 is in chamber 31.

The operation of the device may be summed up as follows:

The fuel system includes primary burners 15 which are operative at all times and secondary burners 16 which are operative at high fuel flows only.

The primary burner nozzle 15 includes an orifice that is designed to require a high fuel pressure to pass the minimum operating fuel flow in order that good atomization is obtained. The fuel system relies primarily on centrifugal pump which is designed for proper operation at high fuel flows when both the primary and secondary burners are in operation and thus when control 14 operates to vary fuel flow in the secondary burner. The centrifugal pump 10 is driven by air turbine 8 to provide the fuel with sufiicient head for delivery into the combustion chamber 41. Under conditions at which all the fuel flow is going through the primary burners 15, the engine will normally be operating at idling speed and probably at a substantial altitude. Under these conditions a centrifugal pump driven by a compressor pressure driven air turbine, will be running at a low speed and consequently be giving a very small pressure rise. However, under these conditions a sufficient pressure head is required in order that a least fuel flow be delivered to the combustion chamber through the nozzle orifice. The low pressure head available from the centrifugal pump is thus incapable of supplying the minimum fuel flow through the specially designed burner nozzle. To overcome this difficulty pump 18 is provided. Pump 13 operates as an auxiliary flow assisting means which is capable of supplying the minimum flow to the nozzles at low engine speeds and high altitude. Since pump 18 is capable of supplying the required flow, the required pressure must be maintained.

It will be clear that with the fuel system of this invention adequate atomization pressure is maintained at the primary injector means 15 under all operating conditions.

If desired, a filter may be incorporated in the line 17a so as to filter the fuel passing to the primary fuel injector means. This filter can be small in relation to the total contamination in the fuel, since the quantity of fuel which is handled by the pump 18 will be of the order of 5 to 10% of the main engine fuel requirements.

We claim:

1. A fuel system for an engine having a combustion chamber to which fuel is delivered continuously in operation of the engine comprising primary and secondary fuel injectors delivering liquid fuel in the combustion chamber, a main pump, control means metering the total fuel delivery of said main pump to said fuel injectors, a main fuel line receiving said total fuel delivery, first and second branch fuel lines leading from said main fuel line to said primary fuel injectors and said secondary fuel injectors respectively, pressurizing means including a constant displacement pump mechanically driven from the engine connected in said first branch fuel line, a non-return valve connected in said second branch fuel line and permitting flow towards said secondary fuel injectors, and means connected between the inlet and outlet of the constant displacement pump permitting delivery of fuel from the outlet to the inlet of said constant displacement pump when said rate of delivery of said constant displacement pump exceeds said rate of delivery to said main fuel line by the main pump, said means permitting delivery of fuel from the outlet to the inlet of the constant displacement pump comprising a recirculating by-pass connecting the inlet and outlet of the constant displacement pump, a valve in the by-pass, and pressure-responsive means operating the valve, the pressure-responsive means being responsive to the pressure of the fuel in said second branch line downstream of said non-return valve which pressure is proportional to the pressure in the combustion chamber when the pressure of the fuel in the main line is insulficient to open the non-return valve and to the pressure at the inlet to the constant displacement pump and being arranged to open the valve when the combustion chamber pressure diifers from the pump inlet pressure by selected amount.

2. A fuel system according to claim 1, wherein said valve in the recirculating by-pass comprises a casing, a valve element movable in the casing and having a pair of spaced lands dividing the casing into two end chambers and a center chamber between the lands, a drilling in one land connecting one end chamber and the center chamber, a connection from the other end chamber to the branch fuel line which leads to the secondary fuel injector at a point in the branch fuel line downstream of the non-return valve, a pair of connections respectively from the inlet and outlet of the pump of the pressurizing arrangement to the center chamber, and a spring urging the valve element towards one end of the casing, the valve element having a position in which the connection from the outlet of the pump is covered and being urged towards this position by the pressure in the center chamber and being urged to uncover this connection by the pressure in said other end chamber.

3. A fuel system according to claim 2, wherein the which the connection from the pump outlet is covered. 5

References Cited in the file of this patent UNITED STATES PATENTS Vogt Sept. 9, 1952 Jubb June 30, 1953 Lee Feb. 23, 1954 Schaffer May 18, 1954 6 Smith July 13, 1954 Torell May 24, 1955 Ballantyne et a1 Dec. 6, 1955 Nims Aug. 7, 1956 Redding Nov. 12, 1957 Ballantyne Nov. 19, 1957 Cruckshank May 27, 1958 FOREIGN PATENTS Australia Feb. 10, 1955 Great Britain J an. 25, 1956 Great Britain J an. 9, 1957 

